Colonography, the use of electronic imaging technologies such as computed tomography (CT) to generate images of a patient's colon for purposes of colorectal cancer screening, is generally known. Descriptions of this diagnostic methodology can, for example, be found in the following U.S. patents, all of which are incorporated herein by reference.
InventorU.S. Pat. No.Vining5,782,762Johnson et al.5,891,030Vining et al.5,920,319Kaufman et al.6,331,116Johnson et al.6,477,401Zalis6,947,784
Briefly, this methodology involves obtaining a series of CT images of adjacent portions or slices of the colon. A radiologist then studies each of the images to identify any pre-cancerous polyps. Also known as virtual colonoscopy, this technique effectively creates a computer simulated intraluminal flight through the colon. This dynamic diagnostic methodology has been demonstrated to be a highly efficacious approach for detecting colorectal polyps.
Although these known colonography approaches are generally much less invasive and more comfortable for the patient that other colorectal cancer screening techniques such as colonoscopy, they sometimes require that the patient's colon be prepared (i.e., cleansed of stool) through the use of laxatives or other cathartics. Removal of the stool is required because the stool exhibits the same density to the imaging processes as the polyps and soft colon tissue. In other words, the stool looks very similar to polyps and the tissues of the colon in the colonography images. The presence of residual stool and fluid can therefore mask polyps and other features in the images that may be relevant to the diagnostic process. Unfortunately, these colon preparation processes can be time consuming and uncomfortable for the patient. Patient compliance with the preparation processes is sometimes poor, resulting in reduced efficacy of the diagnostic procedure. Perhaps even worse, some patients may forego the diagnostic procedure altogether to avoid the inconvenience of the preparation process.
It is known to tag stool with an opacifying agent such as barium prior to CT colonography imaging. The marked or tagged residual stool and fluid is then brighter than soft tissue in the images and recognizable. The tagged stool can also be electronically identified and subtracted from the images. However, accurate stool subtraction is difficult for several reasons. Stool is rarely perfectly labeled, and it is heterogeneous and can contain air pockets with low intensity values. Stool boundaries are also irregular and unpredictable, and partial volume (blurring) effects between stool and air and between stool and tissue will create pixels with intermediate values that are hard to recognize. Furthermore, the effects of noise vary between patients, and can alter voxel intensities significantly.
Stool subtraction after what could be considered partial preparation of the colon is also known. By these techniques the materials ingested by the patient cause the residual stool to be fluid. This generally makes the stool subtraction process easier, since the stool labeling is more homogeneous and the stool-air boundary is generally linear. Unfortunately this preparation approach results in some amount of discomfort and inconvenience to the patient and, possibly, reduced compliance. Accurate stool subtraction on an unprepared colon is typically more difficult.
There is, therefore, a continuing need for improved colonography methodologies. In particular, there is always room for image processing methodologies capable of more accurately identifying and removing residual stool from the images. A methodology enabling improved and accurate stool subtraction on an unprepared colon would be especially desirable. To be viable, any such method should also be efficient to implement. A method that meets these objectives can enhance patient acceptance of the diagnostic procedure, enhance the likelihood of diagnostic accuracy, and thereby reduce the morbidity of colorectal cancer.